Method and apparatus for pre-heating the conductor elements of cables with extruded insulators, in particular conductors with metal tape reinforcements

ABSTRACT

A method and apparatus for preheating the conductor elements of cables with extruded insulator essentially by forced thermal convection. Particularly for conductors with metal tape reinforcement, such as for example Milliken conductors, where it has been found that the traditional magnetic-induction heating is not satisfactory since the tape reinforcement shields the elements of conductors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/EP01/08951, filed Aug. 2, 2001, the content of which is incorporatedherein by reference, and claims the priority of European PatentApplication No. 00202853.8 filed Aug. 14, 2000, the content of which isincorporated herein by reference, and claims the benefit of U.S.Provisional Application No. 60/225,309, filed Aug. 15, 2000, the contentof which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates, in the manufacturing of high-voltage andextra-high voltage extruded cables, to a method and an apparatus forpre-heating the conductor elements of said cables, in particularconductors with metal tape reinforcement, such as for example, Millikenconductors.

The expression “extruded cables” refers to cables wherein the conductorelements are coated with at least one extruded insulating layer.Moreover, in the following and in the attached claims, the term“conductor” shall sometimes be used for the sake of brevity to indicatethe “conductor elements” of the cable as a whole.

2. Description of the Related Art

In their most complete form, high-voltage and extra-high voltageextruded cables comprise internal conductor elements made of strandedcopper or aluminium wires, an internal semiconducting layer (conductorshield), an insulating layer, an external semiconducting layer(insulation shield), a metal shielding consisting, for example, ofhelically wound copper strips and/or wires, extruded lead or analuminium sheet, and optionally an external sheath, made for example ofPVC, extruded polyvinyl or other suitable plastics.

The insulating layer, made—as said—by extrusion, is very critical sinceit is very sensitive to partial discharges that may occur in thepresence of defects such as for example, micro-voids and disjunctionsbetween adjoining layers of materials, which may be present in theinsulation. These partial discharges accelerate the aging of theinsulating material, thus causing its perforation.

Thus, the insulating layer must be as even as possible.

A typical extruded cables manufacturing line comprises a conductorelements pre-heater, a first-stage extruder for the internalsemiconductor, a second-stage extruder for the insulation, a third-stageextruder for the external semiconductor and a triple extrusion head forthe simultaneous coating of the above layers, a heating andcross-linking tube, and finally, a cooling tube to complete thecross-linking process. Alternatively, it is possible to use moreextrusion heads in tandem configuration. Thus, conductors aretraditionally pre-heated just before being introduced into the extrusionhead for the purpose of reducing the temperature difference between theplastic at the melted state and the conductor on which it is extruded.In fact, such a temperature difference causes the formation ofdeformations and similar defects onto the contact surface, which—in thefinal analysis—alter the characteristics of the manufactured cable.

The conductor pre-heating step, moreover, allows obtaining an increasein the plant productivity. In fact, the cable advancement speed must besuch as to allow the complete cross-linking of the “insulated core”,expression that in the present description refers to the conductorelement, the internal semiconducting layer, the insulation, and theexternal semiconducting layer as a whole, that is, the conductor elementafter passing through the extrusion section. The conductor pre-heatingreduces the cross-linking times since the conductor releases heat to—orat least does not absorb from—the extruded material, thus preventing theso-called phenomenon of “freezing” of the internal semiconducting layerand of part of the insulation during extrusion. This phenomenon consistsin that, without conductor pre-heating, the internal semiconductinglayer and the interior of the insulation, when contacting the conductor,release heat to it faster than how they receive heat by conduction fromthe most external layers, so that they fall below the optimumcross-linking temperature. Thus, during the cross-linking step, thereare an external layer being cross-linked, a melted intermediate layerand an internal layer at low temperature. While advancing in themanufacturing line, thanks to the heat received by convection and/orradiation, also the most internal portion, cooled and possiblysolidified, is optionally re-melted, brought back to the optimumcross-linking temperature and cross-linked afterwards. As already said,after the cross-linking there is a cooling step, always in radialdirection, from the outside inwards.

These changes of state and these temperature changes imply that internalstresses are generated in the insulating layer due to the thermalexpansion and contraction, which may worsen the cable performance. Asregards the efficiency of the manufacturing line, it is worsened by thefact that the portion that has cooled down or even solidified due tocontact with the conductor must be heated again or even re-melted, thusthe speed in the cross-linking tube and consequently in the entire plantmust be reduced.

Internal stresses, especially in large cables, may cause a worsening inthe dielectric properties of the insulating layer.

Document JP 61-271717 describes a plant for making a cable with aninsulating resin coating, comprising a feeding drum, driving rollers, apre-heating device, an extruder of resin on the pre-heated conductor, across-linking tube for the extruded resin, driving rollers and a coiler.The pre-heating device, which said document proposes to improve througha device for preventing leakage currents, is based on a system ofcurrent induced by an electrical transformer. Thus, the conductor isheated by the heat generated by Joule effect.

Also direct pre-heating techniques—through electrical current—andinfrared pre-heating techniques have been proposed.

Nevertheless, the Applicant has noted that the pre-heating techniquesmentioned above cannot be satisfactorily applied to conductors providedwith metal—in particular copper—tape reinforcement, such as theso-called Milliken conductors. Milliken conductors, and more in general,lobe-section conductors, are widely used for high-voltage cables as theyexhibit a lower impedance-resistance ratio with respect to equivalentcables of traditional geometry, and they are not so much affected by theso-called skin effect.

As schematically shown in the cross section of FIG. 1, a Millikenconductor 100 has a plurality of sectors or lobes 101, five lobes 101being illustrated as an example in FIG. 1, arranged around a core 102.Core 102, made for example of aluminium, has the purpose of supportinglobes 101 eliminating central points thereof. Each lobe 101 in turnconsists of a plurality of series of wires 103, 104, . . . , 106, 107.Each series of wires 103-107 is helically wound around the more internalseries of wires in the same lobe 101. This multi-lobe geometry forms asubstantially circular cross-section of conductor 100, wherein at thejunctions between the various lobes 101, however, substantiallytriangular grooves 108 are formed along the lenght of conductor 100.During extrusion, the extruded material tends to penetrate into saidgrooves 108, that is to say, it tends to take on an irregularcross-section, not shaped as an annulus (the so-called “fioritura”). Ifthe extrusion occurs at a relatively low pressure, only the internalsemiconductor penetrates into the recesses, but if the extrusionpressure is higher, as in the case of triple-head extrusion section,also the insulator penetrates there, thus causing undesired potentialgradients in the use of the cable.

To obviate this drawback, in addition to imparting mechanical stabilityto the conductors, lobe-section conductors—in particular of the Millikentype—are “tape reinforced”, that is to say, they are wound around with areinforcement tape 109. Said reinforcement tapes, for example, consistof a nylon nonwoven fabric semiconducting layer, a copper layer andanother nylon semiconducting layer.

The Applicant has noted that, in the presence of metal tapereinforcement, the metal absorbs most of the heat provided during theconductor pre-heating, whereas the cable remains cold for the Faradayshield principle: the magnetic field lines only concatenate on thereinforcement tape, which shields the conductor arranged internallythereof from the induction current, thus generating a considerablethermal gradient between the conductor core and the reinforcement tape.

Such a thermal gradient is unacceptable since during the cross-linkingprocess, the inner portion of the conductor, which is colder, removesheat from the reinforcement tape and the insulating material, which arehotter, with the onset the above drawbacks.

SUMMARY OF THE INVENTION

Thus, the technical problem at the basis of the present invention is toprovide a method and an apparatus for pre-heating the conductor elementsfor extruded cables, which should provide a homogeneous pre-heating inradial direction also in the presence of metal tape reinforcement.

Therefore, in a first aspect thereof, the present invention provides amethod for pre-heating the conductor elements of cables provided with atleast one extruded insulating layer, in particular conductor elementswith metal tape reinforcement, comprising the steps of:

a) continuously feeding said conductor elements to a pre-heatingchamber;

b) heating a predetermined flow rate of a thermal carrier fluid to apredetermined pre-heating temperature; and

c) feeding said predetermined flow rate of thermal carrier fluid to saidpre-heating chamber.

In the present description and in the attached claims, the expression“pre-heating temperature” refers to a temperature of the thermal carrierfluid comprised between a temperature immediately higher than that ofthe conductor elements, and a maximum temperature such as to not degradethe polymeric layers laid afterwards onto the conductor elements, anytape present on the conductor elements, or the conductor elementsthemselves. Preferably, the pre-heating temperature of the thermalcarrier fluid is chosen in such a way as to generate a conductortemperature which should be lower than or equal to the extrusiontemperature of the melted polymer, even more preferably, about 10° C.lower than the extrusion temperature. Said temperature can be achievedby suitably adjusting the temperature and/or the flow rate and/or thefluid dynamics characteristics of the thermal carrier fluid. In thisway, the conductor is pre-heated to a temperature capable ofsubstantially reducing the duration of the subsequent step ofcross-linking the layers extruded on the conductor elements.

In parallel, in a second aspect thereof, the present invention providesan apparatus for pre-heating the conductor elements of cables providedwith at least one extruded insulating layer, in particular conductorelements with metal tape reinforcement, comprising:

-   -   a pre-heating chamber suitable to contain a portion of a        predetermined length of the conductor elements and having an        inlet and an outlet for a thermal carrier fluid;    -   a circuit for feeding the thermal carrier fluid towards the        inlet of the pre-heating chamber, and    -   means for heating the gaseous thermal carrier fluid.

Thanks to the achievement of the pre-heating mainly by forced thermalconvection according to the invention, it is possible to effectivelypre-heat both traditional cables and cables with conductor provided withmetal tape reinforcement, for example with Milliken conductor, thuspreventing the problems related to the presence of the reinforcementtape. Moreover, energy consumption is considerably reduced with respectto inductive pre-heating.

Preferably, the thermal carrier fluid is fed to the pre-heating chamberin turbulent condition. In this way, there is an advantageousimprovement in the heat exchange coefficients.

Advantageously, the thermal carrier fluid is counter-current fed withrespect to the direction of continuous feeding of the conductor. Alsothrough this provision, the heat exchange efficiency is improved.

Preferably, the thermal carrier fluid is heated to a predeterminedpre-heating temperature selected in a range comprised between 80° C. and200° C., more preferably between 100° C. and 180° C., and even morepreferably, 130-160° C. This range of temperatures is the bestcompromise between the time needed for the pre-heating and the finalthermal gradient in radial direction of the conductor elements.

Advantageously, moreover, the method of the invention provides fordetecting the conductor element temperature and changing the pre-heatingtemperature and/or the flow rate of the thermal carrier fluid based onthe temperature thus detected. In parallel, the apparatus according tothe invention can further comprise a sensor for detecting thetemperature of the conductor elements, and a controller forautomatically driving the power of the heating means and/or the flowrate of the thermal carrier fluid based on the temperature detected bythe sensor. This feedback control always allows obtaining the desiredtemperature of the conductor elements when entering into the extrusionsection.

In the method according to the invention, moreover, it can be providedto re-circulate, essentially in a closed loop, the thermal carrier fluidfrom an outlet of the pre-heating chamber to an inlet thereof. Inparallel, in the apparatus according to the invention, the circuit forfeeding the thermal carrier fluid can comprise a blower provided withrespective delivery and suction duct extended between the blower and theinlet and outlet for the thermal carrier fluid, respectively. In thisway, the efficiency of the method or of the apparatus, respectively, isfurther improved.

In the apparatus according to the invention, the heating meanspreferably comprises at least an electrical resistor arranged in contactwith the pre-heating chamber, preferably coaxially external thereto.This provides the advantages of construction simplicity andinexpensiveness, besides providing a certain heating of the conductor byradiation.

Preferably, moreover, the pre-heating chamber is closed, at its opposedends, by closing devices having at least one hole for receiving theconductor elements, the hole being movable transversally to itslongitudinal axis. In this way, the hole is movable transversally to thenominal direction of the conductor elements, thus being capable ofadapting itself to the misalignments of the plant and to theconfiguration taken by the conductor elements during the manufacturing,in particular in catenary plants.

Preferably, each closing device has a first plate having a centralprojecting portion wherein said hole is obtained; a second plate closingthe end of the pre-heating chamber and having a slot loosely housing thecentral projecting portion of-the first plate; and at least a thirdplate that can be fastened to the second plate with the first plateinterposed and in offset position with respect to said hole. In fact,such a closing device allows the mobility of the hole although providinga good tightness of the pre-heating chamber.

Even more preferably, the first plate consists of two portions aroundsaid hole, and the second plate consists of two portions around theslot. In this way, the closing devices can be mounted and removed withthe conductor already extended within the plant.

Moreover, the apparatus can be mounted on an adjustable support frame.Said support frame allows the adaptation to the nominal position of theconductor in the plant, particularly in catenary plants.

In a third aspect thereof, the present invention relates to a method formanufacturing a cable provided with at least one extruded insulatinglayer, comprising the steps of:

-   -   pre-heating the conductor elements of the cable according to the        method illustrated above;    -   extruding at least one insulating layer on the pre-heated        conductor elements; and    -   heating and subsequently cooling the insulated core consisting        of said conductor elements provided with at least said        insulating layer for cross-linking at least said insulating        layer.

In a fourth aspect thereof, finally, the present invention relates to aplant for manufacturing a cable provided with at least one extrudedinsulating layer comprising an apparatus for pre-heating the conductorelements of the cable having the described features, an extrusionsection for at least said insulating layer, a cross-linking tube for theextruded layers, and means for continuously feeding the conductorelements.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the invention will now be illustrated withreference to the preferred embodiment, represented by way of anon-limiting example in the attached drawings, wherein:

FIG. 1, which was already referred to, schematically shows a sectionthrough a Milliken type conductor;

FIG. 2 schematically shows a plant for manufacturing an extruded cablehaving an apparatus for pre-heating the conductor elements of extrudedcables according to the present invention;

FIG. 3 shows a side view of a preferred embodiment of a pre-heatingapparatus according to the invention;

FIG. 4 shows an exploded and partially broken-away view of a closingdevice of a pre-heating chamber of the apparatus of FIG. 3;

FIG. 5 shows a view of the closing device of FIG. 4 in the mounted stateand partially broken-away; and

FIG. 6 shows a graph of the results of experimental tests.

DETAILED DESCRIPTION OF THE INVENTION

A plant 1 for manufacturing a cable provided with at least one extrudedinsulating layer, shown in FIG. 2, essentially comprises a pre-heatingapparatus 2, an extrusion section 3, and a cross-linking tube 4,sequentially crossed by a conductor C continuously fed by an unwindingcoil 5, through a so-called delivery “caterpillar” 6. Downstream of thecross-linking tube 4 there are provided a drawing “caterpillar” 6′ and awinding coil 7 for the finished cable. Caterpillars 6, 6′, of course,are only exemplificative of the continuously feeding means of conductorC in plant 1.

The extrusion section 3 is schematically shown as being provided with anextruder 8 for the internal semiconducting layer, an extruder 9 for theinsulating layer, an extruder 10 for the external semiconducting layerand a triple extrusion head 11, but of course, other per se knownconfigurations are possible.

Finally, it shall be noted that, although FIG. 2 shows a plan view of aplant 1 of the horizontal or catenary type, this must not be construedas limiting the invention to said types of plants, as it can be appliedto vertical plants as well.

The pre-heating apparatus 2 according to the preferred embodiment of theinvention, shown in FIG. 3, has first of all a pre-heating chamber 12suitable to contain a portion of length 1 of the conductor elements Cintended for the production of a cable provided with extrudedinsulation, preferably supported within pre-heating chamber 12 by atleast one support 14.

The pre-heating chamber 12 preferably has an elongated tubular shape,for example a stainless steel tube subject to chemical nickel-platingtreatment, and has at its ends tight closing devices 16 of chamber 12,provided with a through hole 18 to allow the passage of the conductorelements C. In view of the length 1 of the pre-heating chamber 12, itcan be made of more portions, as shown by junctions 13 in FIG. 3.

FIGS. 4 and 5 show—respectively in exploded view and in the mountedstate—a preferred embodiment of the closing device 16, wherein thethrough hole 18 is transversally movable, along the double arrows A andB of FIG. 5, with respect to the nominal direction of the conductorelements C in order to adapt itself to the configuration taken by them,which is variable with the horizontal movement of the deliverycaterpillar 6 of plant 1. More in particular, each closing device 16comprises a first plate 161 having a projecting central portion 162,wherein hole 18 is obtained. A second plate 163 has an outer sizecorresponding to the end aperture of the pre-heating chamber 12, andholes 164 for the fastening to it through screws 164′. The second plate163, moreover, has a slot 165 having a slightly larger size than theprojecting central portion 162 of the first plate 161, so as to looselyhouse it. Preferably, and as shown, slot 165 is surrounded by anundercut seat 166 having a size as much larger than the first plate 161as slot 165 is larger than the projecting central portion 162 of thefirst plate 161. At least a third plate 167 (two of them are shown) canbe fastened, for example by screws (not shown) to the second plate 163with the first plate 161 interposed and, in the fastened state, it staysin offset position with respect to hole 18 in any position of the firstplate 161 within the seat 166 of the second plate 163.

For the purpose of allowing mounting and removal of the closing devices16 without removing conductor C from plant 1, the first plate 161preferably consists of two halves 161 a and 161 b around the hole 18,held together by pins 168 (only one is shown in FIG. 4) and by smallplates 169 fixable into undercut seats 170, for example by screws 169′.Similarly, the second plate 163 preferably consists of two halves 163 aand 163 b around slot 165, preferably having a stepwise diametric edge171 for a better tightness. The dashed portions in FIG. 4 illustrate themounting movement of plates 161, 163, around conductor C. Finally, inthe first and second plate 161, 163 there are shown threaded blindholes, respectively 172, 173, for receiving extraction knobs,respectively 174, 175. It must be noted that the rectangular shape ofslot 165 and of the first plate 161, together with the square shape ofthe central projecting portion 162 of the first plate 161, imply adifferent movement capability of hole 18 along the two directions A andB, perpendicular to one another and to the axis of the pre-heatingchamber 12. The closing devices 16 are fastened to the pre-heatingchamber 12, in a catenary plant 1, with such an orientation that themost limited movement in direction A occurs horizontally with respect tothe ground, whereas the widest movement in direction B occurs in thevertical direction. The horizontal movement capability thus provided isparticularly useful for compensating axial misalignment errors duringthe installation of the pre-heating apparatus 2, whereas the verticalmovement capability is particularly useful during operation, so thathole 18 can follow the catenary pattern of conductor C.

Turning back to FIG. 3, the pre-heating chamber 12 is preferablysurrounded by an insulation. 19, for example consisting of a fibreglassor ceramic fibre layer enclosed by an aluminium sheet 20.

Moreover, chamber 12 is provided with an inlet 21 and an outlet 22 for agaseous thermal carrier fluid, for example air, which is fed by a blower24 connected to inlet 21 and to outlet 22 respectively through adelivery duct 26 and a suction duct 28, which are preferably made ofsteel and insulated. The delivery duct 26 and the suction duct 28 arepreferably provided with a respective device 27 and 29 for compensatingthe thermal expansions and contractions, for example in the form ofmetal bellows.

Preferably, as shown in FIG. 3, the air or other fluid circulates in thepre-heating chamber 12 from right leftwards, that is to say,counter-current with respect to the feeding direction of conductors C.This allows improving the heat exchange efficiency, which essentiallyoccurs by forced thermal convection.

In fact, the thermal carrier fluid, which in the pre-heating chamber 12contacts conductor C, is heated through suitable heating means,represented in FIG. 3 as twenty-four half-shell electrical resistors 30arranged along the pre-heating chamber 12, internally of the insulation,through which wiring 31 for feeding resistors 30 are visible. Of course,the “heating means” can be arranged along the entire fluid circuit, thatis to say, also along ducts 26, 28, and it can also be in other forms,for example it can consist of a heat exchanger, which in particular canuse hot fluids obtained from other parts of plant 1.

Nevertheless, it must be noted that the use of electrical resistors 30along the pre-heating chamber 12 allows supplementing the heat exchangeby forced convection with heat exchange by radiation from the walls ofthe pre-heating chamber 12, which in turn are in contact with resistors30, thus improving the efficiency of apparatus 10. Nevertheless, theheat exchange by radiation is negligible with respect to that byconvection, as it has been experimentally proven that it amounts toabout 4%.

Blower 24 can for example be a centrifugal fan.

Moreover, apparatus 2 is preferably provided with a sensor (not shown)for detecting the temperature of conductor C, for example an opticalpyrometer, arranged downstream of the pre-heating chamber 12, prior tothe inlet of the extrusion section 3, and with a controller (not shown)which, on the basis of the temperature detected by the sensor, controlsthe thermal carrier fluid flow rate and/or the current flowing inresistors 30. Preferably, moreover, the controller receives otherparameters in input, provided by suitable sensors, for example thetemperature of the pre-heating chamber 12 and the temperature of the airalong the circuit consisting of blower 24 and delivery and suction ducts26, 28. By way of example, FIG. 3 shows three thermocouples 32 along thepre-heating chamber 12, and a thermocouple 34 along the delivery duct26. Moreover, in the same delivery duct 26 there is shown a hole 36 forreceiving an anemometer (not shown) for controlling the air flow rate.Finally, in the suction duct 28 there is shown a faucet 38 forcompensating the circulating air, normally closed, but that can beuseful during installation of the pre-heating apparatus 2.

Moreover, the pre-heating chamber 12 can be openable so as to facilitatethe insertion and the extraction of conductor C and of the closingdevices 16, or it can be not openable, so as to significantly simplifythe mechanical implementation, thus decreasing costs. In the case of anopenable chamber, it can be suitable for the safety of the personnel incharge to provide for a control over the temperature of the same chamber12 and its effective opening and closing.

Moreover, for the purpose of adapting itself to the configuration takenby conductor C in plant 1, particularly in catenary plants, apparatus 2can advantageously be arranged on an adjustable support frame 40. Thesupport frame 40 comprises chases 41 for supporting the tube which formsthe pre-heating chamber 12. Base 42 of chases 41 is fastened to a frame43 with interposed bearings 44, serving both as thermal insulators andas vibration dampers. Frame 43 is preferably lightened by a series ofholes 45, and has a substantially trapezoidal shape. In its lowerportion, frame 43 is provided with a plate 46, which is fastened to asecond plate 47, constrained to the floor, through a double series ofbolts. Bolts 48 of a first series are each provided with two nuts 49,and they serve for clamping the two plates 46, 47, whereas bolts 50 of asecond series serve for allowing fine adjustment of the slope of theupper side of frame 43, and thus, of the pre-heating chamber 12, so asto be suitable for the insertion in the catenary plant.

With the described apparatus, the method according to the invention canbe actuated as follows.

Firstly, the conductor elements C, which must be pre-heated beforeentering into the extrusion section 3, are continuously fed into thepre-heating chamber 12. At the same time, in the pre-heating chamber 12a predetermined flow rate of air or other preferably gaseous thermalcarrier fluid is heated to a pre-heating temperature, as definedpreviously, so as to heat the portion of conductor C essentially byforced thermal convection.

The thermal carrier fluid is preferably fed in turbulent condition andcounter-current with respect to the continuous feeding direction ofconductor C.

The pre-heating temperature of the thermal carrier fluid and its flowrate are related as follows, in the following simplifying hypotheses:

-   -   the motion of conductor C within pre-heating chamber 12 can be        disregarded since its speed (typically 0.2-0.6 m/s) is much        lower than the speed of the thermal carrier fluid (about 19-38        m/s);    -   the effect of radiation on heat exchange is negligible as it is        equal to about 4% of that provided by forced convection;    -   the thermodynamic properties of air can be deemed as constant        since its thermal head, at steady state, is of just 10° C.

The thermal power Q_(Cu) needed to cause a change ΔT_(Cu) in theconductor is given by the formula of Equation 1:Q _(Cu) =q _(Cu) ·C _(pCu) ·ΔT _(Cu)  (Eq. 1)where q_(Cu) is the feed rate of the conductor and c_(pCu) is thespecific heat of the conductor.

Similarly, the thermal power Q_(f) released by the thermal carrier fluidcan be expressed by Equation 2:Q _(f) =q _(f) ·C _(pf) ·ΔT _(f)  (Eq. 2)where q_(f) is the flow rate of the thermal carrier fluid and c_(pf) isthe specific heat of the thermal carrier fluid.

Considering leaks, the thermal power Q_(req) to be provided to conductorC is given by Equation 3:

 Q _(req) =Q _(Cu) ·K  (Eq. 3)

where K is a constant greater than the unit.

The thermal power exchanged between conductor C and the thermal carrierfluid is a function of the heat exchange coefficient h_(c), of the heatexchange area A, and of the initial and final temperatures of conductorC, and of the thermal carrier fluid. In the case of forced convection inturbulent condition and counter-current flow, one has Equation 4:$\begin{matrix}{Q_{c} = {h_{c} \cdot A \cdot \frac{\left( {T_{Cu} - T_{f}} \right)_{i\quad n} + \left( {T_{Cu} - T_{f}} \right)_{out}}{\ln\frac{\left( {T_{Cu} - T_{f}} \right)_{i\quad n}}{\left( {T_{Cu} - T_{f}} \right)_{out}}}}} & \left( {{Eq}.\quad 4} \right)\end{matrix}$where T_(cu) is the temperature of conductor C and T_(f) is thetemperature of the thermal carrier fluid, where suffix in indicates theinlet of the pre-heating chamber 12, that is, essentially, the outlet 22of the thermal carrier fluid, and suffix out indicates the outlet of thepre-heating chamber 12, that is, essentially, the inlet 21 of thethermal carrier fluid. In the case of heat exchange by convection,coefficient h_(c) is given by Equation 5: $\begin{matrix}{h_{c} = {0,{023 \cdot {Re}^{0,8} \cdot \Pr^{0,4} \cdot \frac{\lambda}{D_{Cu}}}}} & \left( {{Eq}.\quad 5} \right)\end{matrix}$where Re is Reynolds number, Pr is Prandtl number, λ is the thermalconductivity of the fluid, and D_(Cu) is the diameter of conductor C.

By replacing Equation 1 into Equation 3 and equating to Equation 2, andby equating Equations 2 and 4, a two-equation system is obtained. Saidsystem relates the two unknown quantities, flow rate and temperature ofthe thermal carrier fluid at the outlet of the pre-heating chamber 12,essentially at the inlet 21 of the thermal carrier fluid. It has beenproved that, with the flow rate value thus calculated, the stateactually is turbulent as hypotesized, that is Re>2400.

Advantageously, moreover, according to the method of the invention, thetemperature of the conductor elements C can be detected by the sensor,preferably at the outlet of the pre-heating chamber 12, that is to say,after the step of feeding the heated thermal carrier fluid, and thepre-heating temperature and/or the fluid flow rate can be changed bymeans of the controller, based on the temperature thus detected.

Preferably, moreover, the thermal carrier fluid is re-circulatedessentially in a closed loop from outlet 22 of the pre-heating chamber12 to inlet 21 thereof.

After pre-heating, in the method for manufacturing a cable provided withat least one extruded insulating layer according to the invention, atleast the insulating layer is extruded on the pre-heated conductorelements C; then, the insulated core is heated and afterwards cooled tocross-link the insulating layer and any other extruded layers.

By way of example, for pre-heating to about 110° C. a conductor C havinga 1600-mm² section and a 52-mm diameter d, fed at a speed of 0.25-0.6m/min, a thermal carrier fluid flow rate of 0.5-1.5 m³/s will be used,fed at a speed of 19-38 m/s and heated to a pre-heating temperature of140-170° C. In fact, experimental tests have been carried out in suchconditions, using air as gaseous thermal carrier fluid. FIG. 6 shows thepattern of temperature T_(c) (° C.) of conductor C as a function of timet (min), wherein solid line curves represent the temperature detected bya thermocouple arranged at the centre of conductor C, whereas brokenlines represent the temperature detected by a thermocouple arranged atthe periphery of conductor C, halfway its length l. As it can beappreciated from the diagram, the experimental tests have proved thatwith an air temperature of 200° C.—curves 60, 61—the final conductortemperature (indicated by dotted line 62 of FIG. 6) is reached veryquickly (20-25 min), but with a very high gradient in radial direction(the temperature of the conductor surface is about 15° C. higher thanthat of the centre). On the contrary, with an air temperature of about120° C.—curves 63, 64—the temperature gradient at the final temperatureis very low (about 1° C.), but the heating times significantly increase(about 90 minutes). On the contrary, with an air temperature of 145°C.—curves 65, 66—the best compromise is obtained, with a heating time ofabout 45 minutes and a temperature gradient of about 5° C.

It is worth noting that, although the invention is especially applicablefor pre-heating tape reinforced conductor elements, it is advantageousalso in the absence of metal tape reinforcement in terms of energyconsumption. In fact, in the above example the energy consumption of theapparatus is of about 35 kW, of which 5-10 kW for the blower and 1.2 kWfor each of the twenty-four shell-shaped resistors, whereas an inductionpre-heater would require about 80 kW.

It is evident that several modifications, changes, replacements andintegrations can be made to the previously described embodiments withoutthus departing from the scope of the invention, as defined by thefollowing claims.

1. A method for pre-heating conductor elements of cables provided withat least one extruded insulating layer, comprising the steps of: a)continuously feeding said conductor elements to a pre-heating chamber;b) heating a predetermined flow rate of a thermal carrier fluid at apredetermined pre-heating temperature; and c) feeding said predeterminedflow rate of thermal carrier fluid to said pre-heating chamber.
 2. Themethod according to claim 1, wherein said step c) of feeding the thermalcarrier fluid comprises feeding said fluid in turbulent condition. 3.The method according to claim 1, wherein said Step c) of feeding thethermal carrier fluid comprises feeding the fluid counter-current withrespect to the continuous feeding direction of said conductor.
 4. Themethod according to claim 1, wherein said predetermined pre-heatingtemperature is selected in a range between 80 and 200° C.
 5. The methodaccording to claim 1, comprising the further steps of: d) detecting thetemperature of said conductor elements; and a) changing saidpredetermined pre-heating temperature and/or said predetermined flowrate of the thermal carrier fluid based on the temperature detected insaid step d).
 6. The method according to claim 5, comprising the furtherstep of: f) re-circulating, essentially in a closed loop, said thermalcarrier fluid from an outlet of said pre-heating chamber to an inlet ofsaid pre-heating chamber.
 7. A method for manufacturing a cable providedwith a least one extruded insulating layer, comprising the steps of: a)pre-heating the conductor elements of the cable according to thepre-heating method of any one of claims 1-6; b) extruding at least oneinsulating layer on the pre-heated conductor elements; and c) heatingand subsequently cooling the insulated core consisting of said conductorelements provided with at least said insulating layer for cross-linkingat least said insulating layer.
 8. An apparatus for pre-heating theconductor elements of cables provided with at least one extrudedinsulating layer, comprising: a pre-heating chamber suitable to containa portion of a predetermined length of said conductor elements andhaving an inlet and an outlet for a thermal carrier fluid; a circuit forfeeding the thermal carrier fluid to said inlet of the pre-heatingchamber; and means for heating the thermal carrier fluid.
 9. Theapparatus according to claim 8 wherein said heating means comprises atleast one electrical resistor contacting said pre-heating chamber. 10.The apparatus according to claim 8, wherein said circuit for feeding thethermal carrier fluid comprises a blower provided with respectivedelivery and suction ducts extended between said blower and said inletand outlet of the thermal carrier fluid, respectively.
 11. The apparatusaccording to claim 8, wherein said pre-heating chamber is closed, at itsopposed ends, by closing devices having at least one hole for receivingthe conductor elements, said hole being movable transversally to itslongitudinal axis.
 12. The apparatus according to claim 11, wherein eachof said closing devices has a first plate having a central projectingportion wherein said hole is obtained; a second plate for closing saidend of the pre-heating chamber and having a slot loosely housing saidcentral projecting portion of said first plate; and at least a thirdplate that can be fastened to said second plate with the first plateinterpose and in offset position with respect to said hole.
 13. Theapparatus according to claim 12, wherein said first plate consists oftwo portions around said hole and said second plate consists of twoportions around said slot.
 14. The apparatus according to claim 8,further comprising an adjustable support frame.
 15. The apparatusaccording to claim 8, further comprising a sensor for detecting thetemperature of said conductor elements and a controller forautomatically driving the power of said heating means and/or the flowrate of the thermal carrier fluid based on the temperature detected bysaid sensor.
 16. A plant for manufacturing a cable provided with atleast one extruded insulating layer comprising an apparatus forpre-heating the conductor elements of the cable according to any one ofclaims 8-15; an extrusion section for at least said insulating layer; across-linking tube for the extruded layers; and means for continuouslyfeeding the conductor elements.